1.) If you can afford it, get a color headshot. It seems superficial, but these did totally jump out when we were sorting through stacks and stacks of grey headshots.
2.) Make sure your headshot is up-to-date. Plenty of girls with pretty faces submitting for ingenue-type roles and mysteriously wearing Cross-Colors t-shirts and Hammer pants whilst leaning up against posters that read “Free Nelson Mandela” are immediately thrown into the crap pile during a casting. It’s certainly okay to not be 19, but it’s not good to bother casting directors with irrelevant headshots that will make you a disappointment in person.
3.) Include a five word note that shows comprehension of the project you’re applying for, and those casting can’t help but be flattered by the attention you’ve paid. Honestly. I still remember how cool it was to find people who seemed genuininely interested in our project or genre (even if they were just successfully faking). Even a post-it will do.

I would also suggest that people avoid sending insanely hi-rez headshots. During the pinnacle of our casting, I received many oversized headshots that took forever to load, inch by inch. I disregarded many people based solely my impression of their foreheads, for the sake of saving time. Obviously tiny headshots are also bad news. Have a friend who knows something about image editing make you a nice, screen-appropriate, well-compressed jpg- unless you think you’ve got a damn sexy forehead.

-Jon